The present invention is directed toward implantable stimulation electrodes generally, and more particularly to implantable defibrillation electrodes of the types typically used with implantable automatic defibrillators.
Commercially available implantable defibrillation leads typically employ metal electrodes for delivery of defibrillation pulses. In the context of epicardial and subcutaneous leads, the electrode typically takes the form of a mesh of woven metal. A typical electrode of this type is illustrated in U.S. Pat. No. 4,291,707, issued to Heilman et al. In the context of endocardial electrodes, defibrillation electrodes typically take the form of an elongated wire coil, mounted to an insulative lead body of silicone rubber or other material. A typical endocardial defibrillation lead is illustrated in U.S. Pat. No. 4,161,952, issued to Kinney et al.
Implantable defibrillators in clinical investigation, manufactured by Medtronic, Inc., employ subcutaneous and epicardial electrodes in which the metal mesh disclosed in the above electrodes has been replaced by one or more elongated electrode coils mounted to an insulative backing. Electrodes corresponding to those currently in clinical investigation are illustrated in U.S. Pat. No. 4,817,634, issued to Holleman et al, incorporated herein by reference in its entirety. Epicardial and subcutaneous defibrillation electrodes employing elongated electrode coils, but lacking an insulative backing member are disclosed in allowed U.S. patent application Ser. No. 07/604,686 now U.S. Pat. No. 5,105,826 by Smits.
Over the years, the patent art related to implantable defibrillation leads has included a number of references which suggest the use of carbon fibers as a substitute for the wire mesh and conductor coils of epicardial and subcutaneous electrode leads, as disclosed in the patents cited above. References suggesting this substitution include German Patent Application DE 3914662 by Alt and U.S. Pat. No. 4,938,231, issued to Milijasevic et al. Similarly, the use of carbon fibers as conductors or electrodes for pacing leads has also been suggested, for example in U.S. Pat. No. 4,325,389 issued to Gold, and British Patent No. 121017, issued to Thompson Medical Telco.
As a practical matter, the recommendations to incorporate carbon fibers set forth in the patents cited above has not been followed in the context of defibrillation leads. A primary reason is the fact that these leads must deliver extremely high energy pulses. For this reason, commercial embodiments have typically employed metals of extremely low resistivity, such as drawn brazed strand wire for use as a conductor in the lead body and platinum for use as an electrode surface. The problems associated with the use of carbon both to carry the high energy pulses through the lead and to deliver them to the tissue have been addressed to some extent in the above-cited Alt application, in which the use of isotropic carbon is encouraged, as opposed to more typical anisotropic carbon fibers. The use of isotropic carbon fibers is stated to reduce the contact resistance between the individual fibers and to be a benefit in the context of a defibrillation electrode, in that delivery of the current occurs more evenly along the length of the fiber, rather than being focused primarily at the end of the fiber.